This invention relates to polyphosphazenes and to the synthesis of photosensitive compositions useful for etch resistant applications.
Photosensitive compositions and their use as photoresists are well known. Generally positive photoresists compositions are prepared from alkali-soluble, phenol-formaldehyde novolak resins and light or radiation sensitive o-quinone diazides or naphthoquinone diazides. Examples of positive-working photoresists are described in U.S. Pat. Nos. 3,666,473; 4,115,128; and 4,173,470.
Conventional novolak resists have limited etch stability and are not particularly well suited for modern dry etch processes and applications. The advantages of positive photoresists over negative photoresists are also known in the art, such as higher resolution and exposure time stability in the presence of oxygen.
In the industrial application of photoresists the polymeric component and the radiation sensitive component (photosensitizer) are dissolved in an organic solvent or mixture of solvents and applied as a thin film or coating to a substrate suitable for the production of semiconductor devices such as computer chips and integrated circuits.
The polymeric binder resin component of these resist formulations is a film forming material which, for positive photoresists, is desirably soluble in aqueous alkaline solutions, but the photosensitizer acts as a dissolution rate inhibitor with respect to the binder resin. Upon exposure of selected areas of the coated substrate to actinic radiation, the photosensitizer undergoes a radiation induced structural transformation and the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the positive photoresist coating to be dissolved when the substrate is treated with a developing solution, leaving the unexposed areas substantially intact, thus producing a positive relief pattern on the substrate.
In most applications, the exposed and developed substrate will be subjected to treatment by a substrate-etchant solution. The photoresist coating protects the coated areas of the substrate from the etchant and, thus, the etchant is only able to etch the uncoated areas of the substrate. In the case of a positive photoresist this corresponds to the areas that were exposed to actinic radiation. Thus, an etched pattern can be created on the substrate which corresponds to the pattern of the mask, stencil, template, etc., that was used to create selective exposure patterns on the coated substrate prior to development.
The relief pattern of the photoresist on the substrate produced by the method described above is useful for various applications including, for example, as an exposure mask or a pattern such as is employed in the manufacture of miniaturized integrated electronic components or the manufacture of a printing plate.
The properties of a photoresist composition which are important in commercial practice include the solubility of the resist in the application solvent, the photospeed of the resist, development contrast, solubility in an environmently acceptable developer solution, adhesion, dimensional stability at elevated temperature, abrasion resistance, and etch resistance.
Photospeed is important for a photoresist, particularly in applications where a number of exposures are needed, for example, in generating multiple patterns by a repeated process, or where light of reduced intensity is employed such as in projection exposure techniques where the light is passed through a series of lenses and mono-chromatic filters. Thus, high, controlled photospeed is particularly important for a photoresist composition employed in processes where a number of multiple exposures must be made to produce a mask or series of circuit patterns on a substrate. Control of the photospeed is extremely important in order to produce high resolution relief patterns in microcircuitry; e.g., a photospeed which is too high can result in narrowing the range of acceptable processing conditions.
Photoresist resolution refers to the capability of a photoresist system to reproduce the smallest equally spaced line pairs and intervening spaces of a mask which is utilized during exposure with a high degree of image edge acuity in the developed exposed spaces. In many industrial applications, particularly in the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very small line and space widths.
The ability of a photoresist to reproduce very small dimensions of less than one micron, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can be increased, assuming photolithography techniques are utilized, by increasing the resolution capabilities of the resist
Various attempts have been made in the prior art to produce etch resistant positive resists possessing the above desired properties. To date, imagable polyphosphazene resist systems have not been successfully utilized in positive photosensitive compositions. The exceptionally high temperature resistance and flame retardant properties of polyphosphazenes have made them useful as finishes in the fabric industry.
The goal of numerous development programs has been the development of a simple, reliable and cost effective radiation sensitive polymer system that could be used with common photolithographic equipment and processes. However, the silicon-containing polymers which were developed have been plagued by poor shelf life, extraordinarily low photospeed and excessive post-development/post-cure structure contraction. The contraction of original structures upon cure was as much as 60 percent. Although materials based on this chemistry have yielded high resolution structures, they required exposures of ten minutes or longer. Shelf life was also notoriously short, particularly in the high concentrated solutions required for thick film applications.